1. Field of the Invention
The present invention relates to an identity verification apparatus and a fingerprint image pickup method. More particularly, it relates to an identity verification apparatus for and a fingerprint image pickup method of generating a good-quality fingerprint image regardless of the moisture condition on the skin of the fingerprint and when the projections and depressions of the fingerprint cannot be clearly recognized, thereby improving the performance of identity verification.
2. Description of Related Art
In accordance with prior art identity verification apparatus and fingerprint image pickup methods, the fingerprint of a fingertip is brought into contact with a contact surface of a transparent contactable prism, and the back side of the fingertip is irradiated with light emitted out of a light source for lighting. The light incident upon the back side of the fingertip is scattered by the inside of the fingertip which is an outstanding light scattering object, and illuminates the fingerprint from the inner side of the fingertip with uniform light intensity. Since light rays emitted out of the fingerprint ridge portions and the fingerprint valley portions are incident upon the prism at their respective different angles on an interface between the air and the contact surface of the prism, arrangement of an image pickup device in a direction of the light emitted out of the fingerprint ridge portions, i.e., in a direction with an angle larger than the critical angle of total reflection of the interface between the air and the contact surface of the transparent contactable prism can provide a good-contrast picked-up fingerprint image (see Japanese patent application publication (TOKKAIHEI) No. 9-134419 (pages 3 to 4 and FIGS. 1 to 3), for example)
However, since such a prior art identity verification apparatus and a fingerprint image pickup method utilize changes in the angle of light incident upon the prism, the changes depending upon the presence or absence of the air between the contact surface and the fingertip, the degree of contact between the skin of the fingerprint and the contact surface of the prism changes largely according to the condition of the fingerprint side surface of the finger, such as the wet condition of the skin due to sweat, oil, or water, and the condition of the air between the contact surface and the finger also changes according to the condition of the fingerprint side surface of the finger. As a result, the quality of the generated fingerprint image changes according to the moisture condition of the skin of the fingertip, and therefore the performance of identity verification degrades. For example, when the skin of the fingertip is dry, since the projections of the fingerprint are not fully in contact with the contact surface of the prism, the projections of the fingerprint are hard to distinguish from the depressions of the fingerprint in the fingerprint image. In a case where the projections and depressions of the fingerprint cannot be clearly recognized due to wear or the like, there is a possibility that the performance of identity verification degrades and identity verification cannot be carried out.